The real-time Measurement-While-Drilling (MWD) screen showing a display of the pressure pulses sent from the tools downhole (upper left), the data transmission being decoded (lower left), as well as a display of the drilling depth information (lower right), and a display of the various parameters (upper right).

MWD tools are generally capable of taking directional surveys in real time. The tool uses accelerometers and magnetometers to measure the inclination and azimuth of the wellbore at a location, and they then transmit that information to the surface. With a series of surveys at appropriate intervals, anywhere from every 30 ft (10 m) to every 500 ft, the location of the wellbore can be calculated.

The primary use of real-time surveys is in directional drilling. For the directional driller to steer the well towards a target zone, he must know where the well is going, and what the effects of his steering efforts are.

MWD tools can also provide information about the conditions at the drill bit. This may include:
     Rate of penetration
     Rotational speed of the drill string
     Smoothness of that rotation
     Type and severity of any vibration downhole
     Downhole temperature
     Torque and weight on bit, measured near the drill bit
     Mud flow volume

Use of this information can allow the operator to drill the well more efficiently, and to ensure that the MWD tool and any other downhole tools, such as mud motors, rotary steerable systems, and LWD tools, are operated within their technical specifications to prevent tool failure.

A typical MWD tool string with steerable drilling motor.

  MWD / LWD TELEMETRY SYSTEMS
Mud pulse telemetry is the most common method of data transmission used by MWD tools. Downhole, a valve is operated to restrict the flow of the drilling mud according to the digital information to be transmitted. This creates pressure fluctuations representing the information. On surface, the received pressure signals are processed by computers to reconstructs the transmitted information. The technology is available in three varieties - positive pulse, negative pulse, and continuous wave.

Positive pulse tools briefly close and open the valve to restrict the mud flow within the drill pipe. This produces an increase in pressure that can be seen at surface. Negative pulse tools briefly open and close the valve to release mud from inside the drill pipe out to the annulus. This produces a decrease in pressure that can be seen at surface. Continuous wave tools gradually close and open the valve to generate sinusoidal pressure fluctuations within the drilling fluid. Any digital modulation scheme with a continuous phase can be used to impose the information on a carrier signal. The most widely used modulation scheme is continuous phase modulation.

When under-balanced drilling is used, mud pulse telemetry is unusable because a compressible gas is injected into the mud. This causes high signal attenuation which drastically reduces the ability of the mud to transmit pulsed data. In this case, it is necessary to use electromagnetic waves propagating through the formation or wired drill pipe telemetry.

Current mud pulse telemetry technology offers a bandwidths of up to 40 bits per second. The data rate drops with increasing length of the wellbore and is typically as low as 1.5 bps at a depth of 35,000 ft. Data compression is used to increase the effective data rate.

Borehole image log at three different data
 rates: standard mud pulse, accelerated mud
pulse, and wired pipe (left to right) ==>

Electromagnetic telemetry incorporate an electrical insulator in the drill string. To transmit data the tool generates an altered voltage difference between the top part (the main drill string, above the insulator), and the bottom part (the drill bit, and other tools located below the insulator). On surface a wire is attached to the wellhead, which makes contact with the drill pipe. A second wire is attached to a rod driven into the ground some distance away. The wellhead and the ground rod form the two electrodes of a dipole antenna. The voltage difference between the two electrodes is the received signal that is decoded by a computer.

This system generally offers data rates of up to 10 bits per second. In addition, many of these tools are also capable of receiving data from the surface in the same way, while mud pulse-based tools rely on changes in the drilling parameters, such as rotation speed of the drill string or the mud flow rate, to send information from the surface to downhole tools. Making changes to the drilling parameters in order to send information to the tools generally interrupts the drilling process, causing lost time.

Wired drill pipe systems use electrical wires built into every component of the drill string, which carry electrical signals directly to the surface. These systems promise data transmission rates orders of magnitude greater then anything possible with mud pulse or electromagnetic telemetry, both from the downhole tool to the surface, and from the surface to the downhole tool, at data rates upwards of 1 megabit per second,  

Retrievable tools, sometimes known as slim tools, can be retrieved and replaced using wireline through the drill string. This generally allows the tool to be replaced much faster in case of failure, and it allows the tool to be recovered if the drill string becomes stuck. Retrievable tools must be much smaller, usually about 2 inches or less in diameter, though their length may be 20 feet or more. The small size is necessary for the tool to fit through the drill string, however, it also limits the tool's capabilities. For example, slim tools are not capable of sending data at the same rates as collar mounted tools, and they are also more limited in their ability to communicate with and supply electrical power to other LWD tools.

Collar-mounted tools, also known as fat tools, cannot generally be removed from their drill collar at the well site. If the tool fails, the entire drill string must be pulled out of the hole to replace it. However, without the need to fit through the drill string, the tool can be larger and more capable.
 

GEOSTEERING WITH MWD DATA
In the process of drilling a borehole, geosteering is the act of adjusting the borehole trajectory (inclination and azimuth angles) as the well is being drilled, so as to reach one or more geological targets. These changes are based on geological and position gathered from measurement while drilling (MWD) techniques.

Models of underground geological structures are made from available geological data to plan a well trajectory. A well plan is a continuous succession of straight and curved lines representing the geometrical figure of the expected well path. A well plan is projected on vertical and horizontal maps. While the borehole is being drilled according to the well plan, new geological information is gathered from MWD or LWD measurements. These usually show some differences from what was expected from the model. As the model is continuously updated with the new geological information and the borehole position, changes in the trajectory can be initiated to reach the corrected geological targets.

Geosteering and reservoir description (reservoir modeling) are intimately linked. There is no point steering a wellbore if you don't know where to go. The following examples are taken from a recent technical presentation "Horizontal Well Geo-Navigation: Planning, Monitoring and Geosteering" by Rocky Mottahedeh, P.Eng. P.Geol. This paper was presented at the Petroleum Society’s 6th Canadian International Petroleum Conference (56th Annual Technical Meeting), Calgary, Alberta, Canada, June 7 – 9, 2005. (Download).

 
Well trajectory superimposed on a geostatistical model of the reservoir (left) and MWD GR log and well path (right) with FR log of vertical offset well

 
Illustration of steering back into sand after entering shale (left) and actual versus proposed trajectory imposed on reservoir model, GR and ROP help confirm best sand quality (right).